Interpretive Summary: Most invasive weed management programs are geared toward treating weeds to control them. The problem is that weeds are a symptom of some underlying condition or problem that must be addressed if weed management is going to be permanent. Understanding the underlying mechanism promoting weed invasion may ultimately lead to more sustainable solutions. We quantified the role various groups of species have on the invasion of wildland by spotted knapweed. Broadleaved forbs with growth characteristics very similar to the weed were very important in minimizing spotted knapweed invasion. Thus, forbs should be an important consideration during weed management where the invaders are likely to be forbs. We also found that the mechanism for resistance is that desired forbs capture the resources, especially nitrogen, keeping them from weeds that are invading. Among the various groups of species in the community, weed managers may benefit by favoring groups of species with similar traits as that of the invading weeds.

Technical Abstract:
A commonly cited mechanism for invasion resistance is more complete resource use by diverse plant assemblages with maximum niche complementarity. We investigated the invasion resistance of several plant functional groups against the nonindigenous forb Centaurea maculosa (spotted knapweed). The study consisted of a factorial combination of seven functional group removals (groups singularly or in combination) and two C. maculosa treatments (addition vs. no addition) applied in a randomized-complete-block design replicated four times at each of two sites. We quantified aboveground plant material nutrient concentration and uptake (concentration x biomass) use by indigenous functional groups; grasses, shallow-rooted forbs, deep-rooted forbs, spikemoss, and the nonindigenous invader C. maculosa. In 2001, C. maculosa density depended upon which functional groups were removed. ANOVA indicated highest C. maculosa densities occurred where all vegetation or all forbs were removed. Centaurea maculosa densities were the lowest in plots where nothing, shallow-rooted forbs, deep-rooted forbs, grasses, or spikemoss were removed. Functional group biomass was collected and analyzed for N, P, K, and S. ANOVA indicated C. maculosa tissue nutrient percentage and net nutrient uptake were most similar to indigenous forb functional groups. Our study suggests that establishing and maintaining a diversity of plant functional groups within the plant community enhances resistance to invasion. Indigenous plants of the same functional group as an invader may be particularly important in invasion resistance.